As it is known to those skilled in the art, magnetic induction is one of the technologies used to transmit signals between a transmitter and a receiver, which are parts of some wireless communication equipments, for instance.
The magnetic induction is based on the generation of a quasi-static magnetic field component by a first coil of a transmitter, through which is flowing a variable (sinusoidal) current representative of information to be transmitted. When a second coil of a distant receiver intercepts this generated magnetic field, a modulated current representative of the transmitted signals is induced into its winding.
The magnetic field generated by a current loop (or loop antenna) can be divided into three basic terms: one radiation term proportional to r−1 (where r is the distance from the current loop), which represents the flow of energy away from coil, one term proportional to r−2, and finally one quasi-stationary term proportional to r−1.
When the distance between the transmitter and the receiver is small, i.e. when it is smaller than λ/2π (where λ is the wavelength corresponding to the signal frequency), the field propagation is called “near-field mode”. In this mode the quasi-stationary term (1/r3) dominates and is the major contributor. This 1/r3 term is independent of frequency, which implies that it can be employed at any frequency in the near-field mode, for given coil and current, to generate a specified magnetic field at the level of the receiver. So, in the near-field mode, the magnetic field properties are essentially determined by the first coil characteristics, and the electric field is much more weaker than the magnetic field.
In the near-field mode, once the system operating carrier frequency and bandwidth are selected, the transmitter and receiver antenna designs can be optimized independently, with their own constraints of power consumption, size and other design considerations.
In most of the signal transmission systems of the art, the first (transmit) coil is put in resonance with a capacitor with which it forms a LC tank (also called “transmitting (TX) LC circuit”). This decreases the amplitude of the current which drives the antenna (driver current) and reach the required coil current. In receive mode, the circuit front-end being a voltage detector, the induced voltage must be maximized. This can be done by putting the second (receive) coil in resonance with a capacitor with which it forms another LC tank (also called “receiving (RX) LC circuit”).
The TX LC circuit and the RX LC circuit are both tuned to the operating carrier frequency and have both a low quality factor (Q) to pass the majority of the signals through them.
One can show that this antenna tuning is not optimal in terms of power consumption of the transmitter.